Showing papers by "Lodovico Coccato published in 2013"

The planetary nebula population in the halo of M 87

Abstract: Aims. We investigate the diffuse light in the outer regions of the nearby elliptical galaxy M 87 in the Virgo cluster, in the transition region between galaxy halo and intracluster light (ICL). Methods. The diffuse light is traced using planetary nebulas (PNs). The surveyed areas are imaged with a narrow-band filter centred on the redshifted [OIII]λ5007 A emission line at the Virgo cluster distance (the on-band image) and with a broad-band V-filter (the off-band image). All PNs are identified through the on-off band technique using automatic selection criteria based on the distribution of the detected sources in the colour–magnitude diagram and the properties of their point-spread function. Results. We present the results of an imaging survey for PNs within a total eff ective area of 0.43 deg 2 , covering the stellar halo of M 87 up to a radial distance of 150 kpc. We extract a catalogue of 688 objects down to m5007 = 28.4, with an estimated residual contamination from foreground stars and background Lyα galaxies, which amounts to ∼35% of the sample. This is one of the largest extragalactic PN samples in number of candidates, magnitude depth, and radial extent, which allows us to carry out an unprecedented photometric study of the PN population in the outer regions of M 87. We find that the logarithmic density profile of the PN distribution is shallower than the surface brightness profile at large radii. This behaviour is consistent with a model where the luminosity specific PN numbers for the M 87 halo and ICL are different. Because of the depth of this survey we are also able to study the shape of the PN luminosity function (PNLF) in the outer regions of M 87. We find a slope for the PNLF that is steeper at fainter magnitudes than the standard analytical PNLF formula and adopt a generalised model that treats the slope as a free parameter. Conclusions. The logarithmic PN number density profile is consistent with the superposition of two components associated with the halo of M 87 and with the ICL, which have different α parameters. We derive α2.5,halo = (1.10 +0.17 −0.21 ) × 10 −8 N PN L −1 � ,bol and α2.5,ICL = (3.29 +0.60 −0.72 ) × 10 −8 N PN L −1 � ,bol for the halo and the intracluster stellar components, respectively. The fit of the generalised formula to the empirical PNLF for the M 87 halo returns a value for the slope of 1.17 and a preliminary distance modulus to the M 87 halo of 30.74. Comparing the PNLF of M 87 and the M 31 bulge, both normalised by the sampled luminosity, the M 87 PNLF contains fewer bright PNs and has a steeper slope towards fainter magnitudes.

Planetary Nebula Spectrograph survey of S0 galaxy kinematics - II. Clues to the origins of S0 galaxies

Abstract: The stellar kinematics of the spheroids and discs of S0 galaxies contain clues to their formation histories. Unfortunately, it is difficult to disentangle the two components and to recover their stellar kinematics in the faint outer parts of the galaxies using conventional absorption line spectroscopy. This paper therefore presents the stellar kinematics of six S0 galaxies derived from observations of planetary nebulae, obtained using the Planetary Nebula Spectrograph. To separate the kinematics of the two components, we use a maximum-likelihood method that combines the discrete kinematic data with a photometric component decomposition. The results of this analysis reveal that: the discs of S0 galaxies are rotationally supported; however, the amount of random motion in these discs is systematically higher than in comparable spiral galaxies; and the S0s lie around one magnitude below the Tully–Fisher relation for spiral galaxies, while their spheroids lie nearly one magnitude above the Faber–Jackson relation for ellipticals. All of these findings are consistent with a scenario in which spirals are converted into S0s through a process of mild harassment or ‘pestering,’ with their discs somewhat heated and their spheroid somewhat enhanced by the conversion process. In such a scenario, one might expect the properties of S0s to depend on environment. We do not see such an effect in this fairly small sample, although any differences would be diluted by the fact that the current location does not necessarily reflect the environment in which the transformation occurred. Similar observations of larger samples probing a broader range of environments, coupled with more detailed modelling of the transformation process to match the wide range of parameters that we have shown can now be measured, should take us from these first steps to the definitive answer as to how S0 galaxies form.

Signatures of accretion events in the haloes of early-type galaxies from comparing PNe and GCs kinematics

Abstract: We have compared the halo kinematics traced by globular clusters (GCs) and planetary nebulae (PNe) for two elliptical galaxies in the Fornax and Virgo clusters NGC 1399 and NGC 4649, and for the merger remnant NGC 5128 (Centaurus A). We find differences in the rotational properties of the PN, red GC, and blue GC systems in all these three galaxies. NGC 1399 PNe and GCs show line of sight velocity distributions in specific regions that are significantly different, based on Kolmogorov-Smirnov tests. The PN system shows multi-spin components, with nearly opposite direction of rotation in the inner and the outer parts. The GCs velocity field is not point-symmetric in the outer regions of the galaxy, indicating that the system has not reached dynamical equilibrium yet. In NGC 4649 PNe, red and blue GCs have different rotation axes and rotational velocities. Finally, in NGC 5128 both PNe and GCs deviate from equilibrium in the outer regions of the galaxy, and in the inner regions the PN system is rotationally supported, whereas the GC system is dominated by velocity dispersion. The observed different kinematic properties, including deviations from point-symmetry, between PNe and GCs suggest that these systems are accreted at different times by the host galaxy, and the most recent accretion took place only few Gyr ago.We discuss two scenarios which may explain some of these differences: i) tidal stripping of loosely-bound GCs, and ii) multiple accretion of low luminosity and dwarf galaxies. Because these two mechanisms affect mostly the GC system, differences with the PNe kinematics can be expected.

Elliptical galaxies with rapidly decreasing velocity dispersion profiles: nmagic models and dark halo parameter estimates for NGC 4494

Abstract: NGC 4494 is one of several intermediate-luminosity elliptical galaxies inferred to have an unusually diffuse dark matter halo. We use the χ 2 -made-to-measure particle code NMAGIC to construct axisymmetric models of NGC 4494 from photometric and various kinematic data. The extended kinematics include light spectra in multiple slitlets out to 3.5Re, and hundreds of planetary nebulae velocities out to ≃ 7Re, thus allowing us to probe the dark matter content and orbital structure in the halo. We use Monte Carlo simulations to estimate confidence boundaries for the halo parameters, given our data and modelling set-up. We find that the true potential of

Spectroscopic evidence of distinct stellar populations in the counter-rotating stellar disks of NGC 3593 and NGC 4550

Abstract: Aims. We present the results of integral-field spectroscopic observations of the two disk galaxies NGC 3593 and NGC 4550 obtained with the Visible Multi Object Spectrograph at the Very Large Telescope. Both galaxies are known to host two counter-rotating stellar disks, with the ionized gas corotating with one of them. We measured in each galaxy the surface brightness, kinematics, mass surface density, and the stellar populations of the two stellar components, as well as the distribution, kinematics, and metallicity of the ionizedgas component to constrain the formation scenario of these peculiar galaxies. Methods. We applied a novel spectroscopic decomposition technique to both galaxies, to disentangle at each position in the field of view the relative contribution of the two counter-rotating stellar and one ionized-gas components to the observed spectrum. We measured the kinematics and the line strengths of the Lick indices of the two counter-rotating stellar components. We modeled the data of each stellar component with single stellar population models that account for the α/Fe overabundance. Results. In both galaxies we successfully separated the main from the secondary stellar component that is less massive and rotates in the same direction as the ionized-gas component. The two stellar components have exponential surface-brightness profiles. In NGC 3593 they have different scale lengths, with the secondary one dominating the innermost 500 pc. In NGC 4550 they have the same scale lengths, but slightly different scale heights. In both galaxies, the two counter-rotating stellar components have different stellar populations. The secondary stellar disk is younger, more metal poor, and more α-enhanced than the main galaxy stellar disk. Such a difference is stronger in NGC 3593 than in NGC 4550. Conclusions. Our findings rule out an internal origin of the secondary stellar component and favor a scenario where it formed from gas accreted on retrograde orbits from the environment fueling an in situ outside-in rapid star formation. The event occurred � 2G yr ago in NGC 3593 (1.6 ± 0.8 Gyr after the formation of the main galaxy disk), and � 7 Gyr ago in NGC 4550 (less than 1 Gyr after the formation of the main galaxy disk). The formation through a binary galaxy merger cannot be ruled out, and a larger sample is required to statistically determine which mechanism is the most efficient to build counter-rotating stellar disks.

The Planetary Nebula Spectrograph survey of S0 galaxy kinematics - Data and overview

Abstract: Context. The origins of S0 galaxies remain obscure, with various mechanisms proposed for their formation, likely depending on environment. These mechanisms would imprint different signatures in the galaxies' stellar kinematics out to large radii, offering a method for distinguishing between them. Aims: We aim to study a sample of six S0 galaxies from a range of environments, and use planetary nebulae (PNe) as tracers of their stellar populations out to very large radii, to determine their kinematics in order to understand their origins. Methods: Using a special-purpose instrument, the Planetary Nebula Spectrograph, we observe and extract PNe catalogues for these six systems. Results: We show that the PNe have the same spatial distribution as the starlight, that the numbers of them are consistent with what would be expected in a comparable old stellar population in elliptical galaxies, and that their kinematics join smoothly onto those derived at smaller radii from conventional spectroscopy. Conclusions: The high-quality kinematic observations presented here form an excellent set for studying the detailed kinematics of S0 galaxies, in order to unravel their formation histories. We find that PNe are good tracers of stellar kinematics in these systems. We show that the recovered kinematics are largely dominated by rotational motion, although with significant random velocities in most cases. Full Tables 3-7 are available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/549/A115

The Planetary Nebula Spectrograph survey of S0 galaxy kinematics. Data and Overview

Abstract: The origins of S0 galaxies remain obscure, with various mechanisms proposed for their formation, likely depending on environment. These mechanisms would imprint different signatures in the galaxies' stellar kinematics out to large radii, offering a method for distinguishing between them. We aim to study a sample of six S0 galaxies from a range of environments, and use planetary nebulae (PNe) as tracers of their stellar populations out to very large radii, to determine their kinematics in order to understand their origins. Using a special-purpose instrument, the Planetary Nebula Spectrograph, we observe and extract PNe catalogues for these six systems*. We show that the PNe have the same spatial distribution as the starlight, that the numbers of them are consistent with what would be expected in a comparable old stellar population in elliptical galaxies, and that their kinematics join smoothly onto those derived at smaller radii from conventional spectroscopy. The high-quality kinematic observations presented here form an excellent set for studying the detailed kinematics of S0 galaxies, in order to unravel their formation histories. We find that PNe are good tracers of stellar kinematics in these systems. We show that the recovered kinematics are largely dominated by rotational motion, although with significant random velocities in most cases.

Removal of systematics in photometric measurements: static and rotating illumination corrections in FORS2@VLT data

Abstract: Images taken with modern detectors require calibration via flat fielding to obtain the same flux scale across the whole image. One method for obtaining the best possible flat fielding accuracy is to derive a photometric model from dithered stellar observations. A large variety of effects have been taken into account in such modelling. Recently, Moehler et al. (2010) discovered systematic variations in available flat frames for the European Southern Observatory's FORS instrument that change with the orientation of the projected image on the sky. The effect on photometry is large compared to other systematic effects that have already been taken into account. In this paper, we present a correction method for this effect: a generalization of the fitting procedure of Bramich & Freudling (2012) to include a polynomial representation of rotating flat fields. We then applied the method to the specific case of FORS2 photometric observations of a series of standard star fields, and provide parametrised solutions that can be applied by the users. We found polynomial coefficients to describe the static and rotating large-scale systematic flat-field variations across the FORS2 field of view. Applying these coefficients to FORS2 data, the systematic changes in the flux scale across FORS2 images can be improved by ~1% to ~2% of the total flux. This represents a significant improvement in the era of large-scale surveys, which require homogeneous photometry at the 1% level or better.